Drug prescribing is further complicated when patients are put on dialysis because some drugs may be lost in the dialysis bath.15 The extent to which a drug is removed by dialysis is dependent on the drug’s molecular weight, plasma protein binding, volume of distribution, water/lipid solubility, and plasma clearance. In addition, the dialysis of drugs will depend on the technical aspects of the dialysis procedure and the type of dialysis membrane used.
In peritoneal dialysis, the dialysis membrane is the naturally occurring peritoneal membrane; in hemodialysis the membrane is synthetic with fixed pore size. As a general rule, drugs with large molecular weights tend to cross the peritoneal membrane to a greater extent than the hemodialysis membrane. Since only the free drug can cross dialysis membranes, plasma protein binding is an important factor in determining the dialyzability of drugs.
However, plasma protein binding may decrease in uremic plasma increasing the dialyzability of some drugs; and the peritoneal membrane does permit the passage of some proteins leading to some drug-protein dialysis. Conversely, high lipid solubility and low protein binding contribute to a larger volume of distribution, i.e., a drug is widely distributed throughout tissues. Drugs with large volume of distribution are present in plasma in small amounts and are dialyzed minimally.
Drug dialysis data are not readily available for many drugs.3 However, guidelines designed to provide information regarding the dialyzability of drugs is available.15 In general, if hemodialysis and peritoneal dialysis enhances plasma clearance of a drug by more than 30%, supplemental dosing may be required or dosing after dialysis should be considered. If dialysis does not have a clinically important effect on plasma clearance supplemental dosing is usually not required.